1. Field of the Invention
This invention relates to graft copolymers and, in particular, to methods of making graft copolymers such as polysulfone graft copolymers, and articles including such graft copolymers.
2. Description of the Related Art
The ability to control the physical properties of polymers, such as the hydrophilicity, lipophilicity, density, or the molecular weight, is desirable. For example, a polymer that may be desired for a particular application (e.g. a resilient polymer) may not have one or more other physical properties desired, such as wettability; modification of the polymer may achieve the desired properties. For instance, polymers having hydrophilic properties are of great utility, particularly in areas such as resistance to oils and proteins, biocompatibility, resistance to static charge build-up, and wettability to materials such as glues, inks, paints and water. Applications for such polymers include water filtration membranes, biocompatible medical devices and articles. However, most polymers are typically hydrophobic in nature.
One method of producing a hydrophilic polymer is to “coat” a polymer having hydrophobic properties, i.e., covering the polymer with a hydrophilic coating. The coat may be added, for example, by dipping or spraying. However, the coated areas are generally not permanently attached to the underlying polymer support.
Another technique of producing a hydrophilic polymer is to synthesize a polymer having a graft structure. A “graft copolymer” is produced by covalently bonding a species to be grafted (also referred to as a “comonomer”), to a backbone polymer. Graft copolymers may be used to provide a material having specific properties while retaining certain desirable properties of the original backbone polymer.
The production of graft copolymers has previously been achieved using two general techniques: photochemical and chemical. Photochemical techniques include exposure of a polymer to low-temperature plasma, ultraviolet irradiation, or gamma-ray irradiation to begin the polymerization process. Chemical techniques typically use free-radical polymerization or atom transfer radical polymerization to produce the grafted copolymer. For example, Mika, et al., “A New Class of Polyelectrolyte-Filled Microfiltration Membranes with Environmentally Controlled Porosity,” J. Memb. Sci., 108, 37-56 (1995) describe UV-induced grafting of 4-vinylpyridine onto polyethylene and polypropylene microfiltration membranes. Iwata, et al., “Preparation and Properties of Novel Environmental-Sensitive Membranes Prepared by Graft Polymerization Onto a Porous Membrane”, J. Memb. Sci., 38, 185-199, 1988) report a glow discharge technique to graft polyacrylamide and polyacrylic acid chains onto polyvinylidene fluoride (PVDF) membrane. Hautojarvi, et al, (J. Memb. Sci., 108, 37, 1995) published a similar study of PVDF membranes graft-modified with poly(acrylic acid). However, in many cases, the radicalization process is difficult to control, and unwanted side reactions such as degradation or cross-linking can occur, as well as difficulties in minimizing polydispersity. Also, in many cases a bulk polymer article is produced, and then functional groups are grafted onto polymer chains that define the article. This can result in non-ideal characteristics. Another graft copolymer synthesis technique is described in U.S. patent application Ser. No. 09/951,125, filed Sep. 12, 2001, by Mayes et al.
While the above and other reports include, in many cases, useful graft copolymers, there is a continued need in the art for impoved, more versatile graft polymerization techniques and articles produced by these techniques.